4.2 Differences Between Unix and Windows

Unix and Windows use completely different paradigms for run-time
loading of code. Before you try to build a module that can be
dynamically loaded, be aware of how your system works.

In Unix, a shared object (.so) file contains code to be used by the
program, and also the names of functions and data that it expects to
find in the program. When the file is joined to the program, all
references to those functions and data in the file's code are changed
to point to the actual locations in the program where the functions
and data are placed in memory. This is basically a link operation.

In Windows, a dynamic-link library (.dll) file has no dangling
references. Instead, an access to functions or data goes through a
lookup table. So the DLL code does not have to be fixed up at runtime
to refer to the program's memory; instead, the code already uses the
DLL's lookup table, and the lookup table is modified at runtime to
point to the functions and data.

In Unix, there is only one type of library file (.a) which
contains code from several object files (.o). During the link
step to create a shared object file (.so), the linker may find
that it doesn't know where an identifier is defined. The linker will
look for it in the object files in the libraries; if it finds it, it
will include all the code from that object file.

In Windows, there are two types of library, a static library and an
import library (both called .lib). A static library is like a
Unix .a file; it contains code to be included as necessary.
An import library is basically used only to reassure the linker that a
certain identifier is legal, and will be present in the program when
the DLL is loaded. So the linker uses the information from the
import library to build the lookup table for using identifiers that
are not included in the DLL. When an application or a DLL is linked,
an import library may be generated, which will need to be used for all
future DLLs that depend on the symbols in the application or DLL.

Suppose you are building two dynamic-load modules, B and C, which should
share another block of code A. On Unix, you would not pass
A.a to the linker for B.so and C.so; that would
cause it to be included twice, so that B and C would each have their
own copy. In Windows, building A.dll will also build
A.lib. You do pass A.lib to the linker for B and
C. A.lib does not contain code; it just contains information
which will be used at runtime to access A's code.

In Windows, using an import library is sort of like using "import
spam"; it gives you access to spam's names, but does not create a
separate copy. On Unix, linking with a library is more like
"from spam import *"; it does create a separate copy.